Functional differentiation of alveolar type II epithelial cells in vitro: effects of cell shape, cell-matrix interactions and cell-cell interactions

Abstract

Alveolar type II epithelial cells rapidly lose characteristics of differentiated function when cultured on plastic dishes. We have attempted to circumvent this problem by culturing type II cells under conditions that might better reproduce their environment in vivo. Cell-matrix interactions were studied by culturing isolated adult rat type II cells on Engelbreth-Holm-Swarm (EHS) tumor basement membrane. Aggregates of type II cells formed on the surface of the matrix during 4 days in culture. Microscopic examination of these aggregates revealed cuboidal cells that retained more characteristics of differentiated type II cells than did cells cultured on plastic. Type II cells cultured on EHS matrix incorporated a higher percentage of acetate into phosphatidylcholine (PC) than did cells on plastic, and a higher percentage of this PC was saturated. Phosphatidylglycerol (PG) synthesis by these cells was no different from that seen in cells on plastic. The effects of cell-cell interactions and cell shape were evaluated by culturing type II cells on feeder layers that in turn were grown on collagen gels. The feeder layer cells included fetal rat lung fibroblasts, adult rat lung fibroblasts, fetal rat skin fibroblasts, bovine aortic endothelial cells, and rat mammary tumor epithelial cells. One-half of the gels remained attached to the culture dish and one-half of the gels were detached after 24 h and allowed to float free in the medium. Type II cells grown in association with any of the attached feeder layers became flattened and lost their differentiated phenotype. These cells incorporated no greater percentage of acetate into PC than did cells on plastic. Saturated PC synthesis was modestly increased. PG synthesis declined in parallel with that seen in cells cultured on plastic. Type II cells cultured on feeder layers that were detached assumed their native cuboidal shape and also exhibited many morphological characteristics of differentiated function. These cells incorporated a significantly greater percentage of acetate into PC compared to cells on either plastic or attached feeder layers. Saturated PC synthesis also increased markedly. These cells, however, incorporated no greater percentage of acetate into PG than did cells on plastic or attached feeder layers. These data suggest an important role for cell shape and cell-matrix interactions and maintenance of type II cell differentiation. The effects of cell-cell interactions, while beneficial, appear to be non-specific.